As shown in FIG. 1, a cycloconverter is a device which is used to convert three phase input power at line voltage and frequency to three phase output power at a different, controlled voltage and frequency. Typically, a cycloconverter is used to convert the line voltage waveforms to those desired to be applied to the rotor of a variable speed generator so that the output from the generator matches the line voltage waveforms. A common 6 pulse cycloconverter design typically includes six bridge circuits, two for each phase. Each bridge circuit typically contains six switching devices. There are other implementations of a cycloconverter such as a 12 pulse design which used twice as many switching devices as the 6 pulse design, however, the underlying operating principles are the same.
The output of the cycloconverter is controlled by adjusting the timing of the firing pulses for each of the switching devices. The maximum output of the cycloconverter is obtained if each switching device is fired at a time called the "natural commutation time," i.e., the time at which the switching device would begin to conduct if it were an ideal diode. If the firing time of the switching devices is delayed, the output of the cycloconverter is modified by the cosine of the phase delay.
Modulation of the time delay can be used to form any desired output waveshape. The appropriate modulation results in the desired output waveform being applied to the rotor. The firing times of the switching devices in a cycloconverter are typically controlled by a gate pulse generator. A gate pulse generator generates timing waveforms and compares these to a set of three reference voltages (one for each phase) and fires the appropriate switching devices when these signals are equal.
Analog gate pulse generators have been developed and used for this purpose; however, they have several significant disadvantages. These disadvantages include the use of a large number of analog circuits. A typical analog gate pulse generator can require a large amount of control circuitry consisting of four "Multibus" boards and an auxiliary input panel. It also requires extensive additional control circuitry to implement certain desired functions such as "end stop" and "output pulse shaping."
Analog gate pulse generators also require numerous precision components to provide the desired operating accuracy. These components also require periodic adjustment and are subject to degradation in performance due to component drift from aging or changes in temperature.
An analog gate pulse generator also requires three line voltage inputs for each phase of the output voltage (a total of nine inputs) and two differential voltage inputs for each phase of the output voltage (a total of six outputs). A significant amount of the total wiring of the generator control system is devoted to developing these voltage inputs. It is also believed that a significant amount of the undesirable noise on the output of a variable speed generator at slip frequency is due to inaccuracy in the analog gate pulse generator.
Previously known digital gate pulse generators for controlling cycloconverters are disclosed in U.S. Pat. No. 4,017,744; Park et al, "Microprocessor--Controlled Cycloconverter," Proceedings of the 1979 International Symposium on Circuits and Systems, Tokyo, Japan, July 1979, pp. 942-43; Tso et al, "Efficient Microprocessor-based Cycloconverter Control," IEEE Proceedings, Vol. 127, Pt. B, No. 3, May 1980, pp. 190-96; and Singh et al, "Microcomputer-Controlled Single-phase Cycloconverter," IEEE Transactions On Industrial Electrics and Control Instruments, Vol. IECI-25, No. 3, August 1978, pp. 233-38. The devices in these references, however, do not respond fast enough to changes in the output demand.
Other cycloconverter control systems are disclosed in U.S. Pat. Nos. 3,585,485; 3,858,105; 3,982,167; 4,225,911; 4,349,867 and 4,356,542.